Vitamin A is one of the fat-soluble vitamins, which means that you will absorb it better if you eat it with some fat. It also means that your body stores it in a different way than the non fat-solubles so there is a risk for overdose which wouldn't good. It can for example give birth defects so pregnant women should stay away from (example) liver as that contains a lot of this vitamin.
There are different types of vitamin A that I won't go to deep into here. We get most of our vitamin A in a form called esterified retinol and from the provitamin β –carotene.
Carotenoids (for example carotene) turns into vitamin A in the body. That is the red/yellow pigment you can find in green vegetables (they are hidden by the green chlorophyll) or for example carrots. You absorb about 10-20 % of β –carotene and it then helps if you heat it up a little bit - the chemical structure will then change so you will be able to absrob it (to put it simply). That means that some vegetables are better to heat up then eating raw when it comes to carotenoids.
We have a protein in us that our liver produces itself that transports the vitamin to the different cells that needs it. A lack of this specific protein would give us a vitamin A deficiency as the transport of the vitamin wouldn't work. We also need zinc for the release of the protein from the liver. When the protein has released the vitamin then it will go to our kidneys to be destroyed - the liver always have to continue produce this specific protein.
90 % of our vitamin A storage are in the liver.
We need it for our vision, reproduction, epithelial tissues normal differentiation (very simply translated - for the walls of our intestines) and our immunsystem.
You will find it in (examples) organs, fats, vegetables, root vegetables.
If you heat it up too strongly or a long time then the vitamin will be destroyed as the chemical structure changes so we won't be able to use it. The same thing happens in prolonged storage at room temperature. It is also affected by light, oxygen and low pH (in a bad way).
It has been shown in studies on animals that it has prevented cancer, but if you are a smoker and eat β –carotene supplements then there is a higher risk for lung cancer.
Source: Näringslära för högskolan (Nutrition for higher studies)